David Cameron should show no mercy in the by-election to replace Chris Huhne

How should David Cameron approach the Eastleigh by-election? The Hampshire seat brings bitter memories to anyone who was involved in the party in 1994 (and to those of us who spent time on the Lobby by-election trail back then). It started badly with the bizarre death of the former journalist Stephen Milligan. And it finished in disaster for the Conservatives, with a big swing of nearly 17pc to the Lib Dems and a thumping defeat for John Major, one of many in a parliament that saw the gradual erosion of his majority down to nothing.

You could hardly think of better circumstances to test the Coalition: the resignation of Nick Clegg's deadly rival and great Tory hate-figure; a seat in the crucial south-coast battleground between Tories and Lib Dems, one heavy with memories of brutal warfare between the two parties; a by-election so soon after the bitterness of the boundaries vote that left Tories thirsting for revenge and Nick Clegg worried that they will come for him. All in all, just perfect.

There might have been a time when voices would suggest an arrangement – a coupon even – to keep the Coalition all sweetness and light. In its earliest days you might just have imagined Nick Boles or even Michael Gove urging Mr Cameron to put up only a token effort, and allow his Coalition partner a free run at retaining the seat. Not now. Tory voices will be unanimous in urging an all-out effort to seize this seat. And Mr Cameron should heed them. He should order his party to throw everything at winning Eastleigh.

His personal situation is sufficiently precarious inside the party to make defeat downright dangerous. The same applies with the voters, who need to see him winning for once. If his story about difficult choices to help Britain win the global race is to be judged persuasive, he needs to show it works in the southern heartlands. If Mr Cameron cannot win in Eastleigh, his MPs will say, he cannot win in 2015. That's not true, of course: mid-term by-elections should never be treated as anything other than a safe way to send a message to an unpopular government. All the usual caveats apply. But Mr Cameron needs to win this one. Just as Mr Clegg argued that the boundaries vote was a self-contained piece of necessary but unpleasant business, so Mr Cameron should tell him that, in Eastleigh, the gloves are off.